Americans may be tightening their belts. Or at least not expanding them.

The percentage of adults who are obese hasn't increased much over the past 10 years after several decades of skyrocketing growth, an indication that America's obesity epidemic is finally starting to level off, according to a landmark government analysis released Wednesday. About 34% of U.S. adults — almost 73 million people — were obese (roughly 30 or more pounds over a healthy weight) in 2008, up from 31% in 1999. "The obesity trend appears to be slowing down, but the prevalence remains high and continues to be a critical national health concern," says Cynthia Ogden, an epidemiologist with the National Center for Health Statistics, part of the Centers for Disease Control and Prevention (CDC).

Donna Ryan, president of the Obesity Society, a group of weight-loss researchers and professionals, says she believes it may have plateaued for now, but "to level off at 34% obesity is no great achievement. It's still very, very alarming. And the high rates of childhood obesity are likely to translate into higher rates of extreme obesity when those children reach adulthood."

Extra weight increases the risk of diabetes, heart disease, some types of cancer and other health problems. Americans who are obese cost the country an estimated $147 billion in weight-related medical bills in 2008.

Obesity was relatively stable in the U.S. between 1960 and 1980 when about 15% of people fell into the category, and then it increased dramatically in the '80s and '90s.

Ogden has done other research showing the leveling off of obesity for women and children over the past few years. For this study, she and colleagues compared weight trends in adults and children between 1999 and 2008 and published the findings online in the Journal of the

American Medical Association:

•32.2% of men are obese, up from 27% in 1999, but it has been at the current level since 2003.

•35.5% of women are obese compared with 33.4% in 1999, a slight increase that is not statistically significant.

•Overall, about 31.7% of kids are obese or overweight, compared with 29% in 1999, also not a statistically significant difference.

William Dietz, director of the CDC's Division of Nutrition, Physical Activity and Obesity, says this may reflect that people are becoming aware of "the adverse health consequences of obesity" and are adopting healthier habits. "The plateau in tobacco use began when people became aware of the health problems associated with its use," he says. "We're at the corner with obesity, but we haven't turned the corner. Turning the corner would mean we would see a decline."

This analysis is based on data from the National Health and Nutrition Examination Survey, which is considered the gold standard for evaluating the obesity problem in the USA because it is an extensive survey of people whose weight and height are actually measured rather than being self-reported.

Obesity in adults is defined as a body mass index (BMI) of 30 or greater. Children are classified as overweight or obese based on where they fall on BMI growth charts. Some other trends in weight:

• Obesity is more common among women who are black (49.6%) and Hispanic (43%) than white (33%).

•Obesity is greater than 30% in most age groups for both sexes, except for men ages 20 to 39.